Pokérus is a virus in the Pokémon series that any Pokémon can catch if it has not already been infected once before. Its name derives very clearly, from "Pokémon" and "virus."

Description

Pokérus is caught when a Pokémon battles a wild Pokémon. The effects of this virus are very beneficial to the Pokémon that receives it. Catching the virus however, is a very rare event, as the odds of meeting a wild Pokémon with it is 1 to between 21,178 and 21,845, or roughly 0.00004651%. Similar to the Macho Brace, Pokérus will double the Effort Value stats of a Pokémon. A player can combine a Macho Brace and Pokérus to get a substantial amount of EV's.

When caught, the Nurse Joy in a Pokémon Center will inform you if you have received it, saying, "Oh... It looks like your Pokémon may be infected with the Pokérus. Little is known about the Pokérus, except that it is a microscopic life-form that attaches to Pokémon. While infected, Pokémon are said to grow exceptionally well.". Also, in HeartGold and SoulSilver, Professor Elm will call you and say something around, "You got the Pokérus. Don't worry, it will wear off."

The Pokérus is spread through training and battling Pokémon who do not have Pokérus with a Pokémon who does have it. The virus does not last forever; after two days or so, it will wear off of the Pokémon and will not be able to be received again. However, if placed in the PC Box, Pokérus will last until you take it out.

After the timer has run out for Pokérus, you still get the benefits. However, you don't get to spread it around. It is advisable to try and keep Pokérus by putting the infected Pokémon into PC Box before midnight (12:00 AM), as the virus only lasts one to four days.

Pokémon with the Pokérus will lose Pokérus eventually, and a small pink smile will appear in your Pokémon's status screen. The small pink smile on your Pokémon's status means that it has already had Pokérus, and cannot get it again.

Strategy

Have the Pokémon with Pokérus in the middle (third slot). Keep on battling until your entire team gets Pokérus. If the last Pokémon in your party did not get Pokérus, simply switch it so that it is closer to the rest of your Pokémon. It is always a good idea to have at least three Pokémon with Pokérus in your PC Box. Pokérus can spread to different Pokémon in your team. Pokémon with Pokérus are often received by other Pokémon in a trade.